1. Field of the Invention
The present invention relates to a switching regulator that performs voltage conversion by switching with a pulse width modulation signal (hereinafter referred to as a PWM signal), and in particular to a digital control switching regulator having an input voltage detection circuit and suited for control with a digital signal.
2. Description of the Related Art
In switching regulators in general, controller coefficients of a voltage conversion circuit are desired to be set at optimum values corresponding to variation of an input voltage and a load current. For carrying out the controller coefficient optimization, a control mode made by an analogue circuit has a problem in that the analogue circuit requires preparation of a hardware circuit device corresponding to setting values of the controller coefficients, and such a circuit is typically complicated and large-sized.
Patent Document 1 discloses a switching regulator in a digital control mode to provide a technology to avoid the enlarged circuit size. FIG. 8 shows an example of construction of a conventional digital control switching regulator, which is disclosed in Patent Document 1.
Referring to FIG. 8, input ac power AC is, through an input filter 21, fed to a rectifying and smoothing circuit 22 composed of a full-wave rectifying diode circuit 23 and a capacitor Ci, and supplied to a transformer unit 24 with a switching circuit. The transformer unit 24 with a switching circuit performs ON/OFF control by a switching element QN on the supplied power from the rectifying and smoothing circuit 22. The ON/OFF output is coupled with an output circuit 26 through a transformer 25. The output circuit 26 consisting of diodes D1, D2, inductor L, and capacitor Co converts the switching output that is supplied by the transformer unit 24 with a switching circuit to a dc voltage and outputs the converted output. The output voltage Vout is supplied to a load circuit 8.
The output from the output circuit 26 is taken out as an output voltage analogue signal AS1, and the output from an output current detection circuit 27 is taken out as an output current analogue signal AS2. The output voltage analogue signal AS1 and the output current analogue signal AS2 are fed to a digital control unit 28. A control output from the digital control unit 28 is fed to the switching element QN, a component of the transformer unit 24 with a switching circuit. The switching element QN operates to determine a duty ratio, a ratio of ON time to OFF time in a switching period.
The digital control unit 28 converts the input signals of the output voltage analogue signal AS1 and the output current analogue signal AS2 into an output voltage digital signal and an output current digital signal, and executes operation of a command value for determining the duty ratio of the transformer unit 24 with a switching circuit based on the output voltage digital signal and the output current digital signal together with input voltage information or load terminal voltage information. The input voltage information or the load terminal voltage information is estimated from the output voltage digital signal and the output current digital signal.
The digital control unit 28 generates pulses based on the command value obtained by the operation and controls switching operation of the transformer unit 24 with a switching circuit by the fed pulses, thereby controlling an output voltage and an output current.
The input voltage information necessary for executing the command value operation is estimated by the following formulas (1), (2), and (3). An equation in a discrete variable system applied to a digital processing in FIG. 8 is represented by Formula (1) below:[Formula 1]Vi(n−1)=L[i(n)−i(n−1)]/Tc+Vo(n−1)  (1)wherein Vi(n) is a mean voltage between a time (n−1) and a time n at a position indicated in the output circuit 26 in FIG. 8, L is an inductance, i(n) and i(n−1) are inductor current values at a time n and a time (n−1), respectively, Tc is a sampling period, and Vo(n−1) is an output voltage at a time (n−1).
Provided the sampling period Tc be k times the switching period T (k is an integer) and an ON time duration of the switch be Ton in the switching period T, the relation represented by the following Formula (2) holds between the mean voltage Vi(n−1) and the input voltage V(n−1).[Formula 2]Vi(n=1)=k·Ton·V(n−1)/Tc  (2)From Formula (1) and Formula (2), an actual current i(n) at the time n is given by the following Formula (3).[Formula 3]i(n)=i(n−1)+Tc·[k·Ton·V(n−1)/Tc−Vo(n−1)]/L  (3)
Thus, Patent Document 1 states that the optimum control can be carried out corresponding to variation of an input voltage and an output current by operational estimation of the input voltage based on the formulas (1), (2), and (3) using the detected values of output voltage analogue signal AS1 and output current analogue signal AS2.    [Patent Document 1] WO97/50165.
The above-mentioned conventional digital control switching regulator, however, has the problems as follows.
The digital control unit 28 performs the control by executing an operation for the command value to determine the duty ratio of the switching regulator based on the detected values of output voltage analogue signal AS1 and output current analogue signal AS2. However, since the input voltage information used for executing the operation is an estimated value based on the output voltage and the output current, there is a problem of causing an error. An inductance value, which is necessary for the estimation operation, is generally decided by the user side and thus, an uncertain factor. Since the control is conducted by the operation that uses an estimated value and an uncertain factor, the digital control switching regulator cannot perform optimum control.